how have you soldered the mosfet up ????
if you have used the std wiring with the mosfet that is....
now - if using std wiring you remove one of the old red/posi leads and place BOTH on same terminal
then the signal wire only goes to the old terminal.....
the motor now has the positive permanently running to it with the join/link of them two wires you did at switch
the mosfet has the negative running to it and out the other side to the motor
the thin signal wire runs to mosfet from trigger - just that wire on that contact...
oh f*ck it pic says 1,000 words.....
or have you rewired it for dedicated wires to motor & 2 very thin supply/signal wires at switch
normal method if doing a total rewire say on a rear wired gun for example:
ignore the type of mosfet non ab or ab it makes no difference - the wiring method(s) really depends on installer and if using existing or new rewire methods plus personal choice and what have you - same meat different gravy.....
if you was building your own 3034 the red/posi would run straight to motor but with a thin wire wrapped around it taking a posi supply to trigger so on some builds it may look like a positive wire is totally uninterupted to motor - well yeah it is but with a thin wire tapping into it with shrink wrap blah blah blah....
at very first guess I just need you to double/triple check your wiring is correct and no shorts
it should not make no difference in auto/semi and certainly not start shooting on its own in just one of those positions unless something is shorting or wired incorrectly
even if mosfet is permanently on it should not matter in semi/auto - they should both not operate until trigger pulled
you may need a test meter but something is way out of whack
My guess is that you are using the first "existing" method and need to ensure the two 16awg wires are on same terminal on switch
double/triple check that there is not a tiny whisper hair bridging those two wires to old terminal. Now the thin mosfet signal wire goes on the bare terminal - again double check nothing is bridging the trigger switch terminals....
them prongs at rear matter nothing - they are bridged just in case the copper plate doesn't make full 101% contact on them all the time
still it should not fire in semi or auto coz the circuit is not closed until the trolley - slidey lug bit presses in and touches both contacts up top thus completing the circuit
you must have a weird short or wiring issue - the mosfet will either work or not in both semi/auto exactly the same
ps - did you build your own mosfet or one off the shelf ???
there are numerous fake 3034's on fleabay that are just pure $hite
if building your own then get a m8 or two to go in with ya and buy 10 from RS or something:
http://uk.rs-online.com/web/p/mosfet-transistors/6887204/?origin=null|fp&cm_sp=featureproducts-_-FeaturedProductsContent-_-6887204
or
a great cheap mosfet ready to go is firestorm:
http://www.firestormsystems.com/shop/electronics/mosfet-switch.html
that is £10 delivered with thermal fuse built in - I don't recommend much but this baby is blinding deal with wires n stuff for anybody just needing one or two basic mosfets
if I can help lmk coz reckon it just a really daft issue you are having sir